London, Dec. 8: Scientists believe they may have discovered why left-handers are so common, despite suffering disadvantages when it comes to handling tools, disease risk and historical prejudice in predominantly right-handed societies against the 'cackhanded'.

Because this trait is substantially inherited, and because it can be a disadvantage, scientists have puzzled for years over what it is about being left-handed that helps survival, or the ability to reproduce.

Now it seems that left-handers are more likely to thrive in a violent society. A French team today reported in the Proceedings of the Royal Society: Biological Sciences that because they are in the minority, left-handers have a strategic advantage in fights.

The reason is not that they are innately superior but that their opponent is likely to be more used to combat with right-handers, according to Charlotte Faurie and Michel Raymond of Universit' Montpellier II, France.

The team was inspired to carry out the study by the observation that, in a right-handed society, left-handers have an advantage in sports such as fencing, tennis, cricket and baseball.

They reasoned that 'interactive sports in western societies are special cases of fights, with strict rules, including in particular the prohibition of killing or intentionally wounding the opponent'.

Thus it could be that being left-handed in a right-handed society may have offered an advantage in fights. 'If this is true, then the advantage of being left-handed should be greater in a more violent context, which should result in a higher frequency of left-handers.' They decided to test their idea.

Earlier studies have shown a number of left-handers in the Kreyol people of Dominica, the Ntumu people of Cameroon, the Dioula-speaking people of Burkina Faso, Baka people of Gabon, Inuit people, and the Eipo people in Irian Jaya, New Guinea.

When the team studied the rate of murder in each society, they found 'a significant positive correlation between homicide rates and left-handedness frequencies'.

The Dioula had a homicide rate equivalent to one hundredth of a death per 1,000 people per year, while the Eipo had around three deaths per 1,000 people. And the percentage of left-handers was 3.4 per cent and more than 20 per cent, showing how left-handedness thrived along with aggression.

In societies, being able to win fights meant more than killing opponents, enabling warriors to gain status, and impress women.

A study by the same team of 600 students found that the same forces are at work today in sports, which they describe as 'ritualised fights'. Competitive athletes, particularly men, have more sexual partners than their couch-potato peers. Because left-handers often have a competitive advantage in sports, they are more likely to enjoy this benefit, said Faurie.

However, left-handers face disadvantages. Statistics suggest reduced life expectancy, smaller body size and immune system disorders, she said. 'But it seems to be true that they are more frequent among some professional categories as musicians and mathematicians,' said Dr Faurie.

'Our team showed that they have some socio-economic advantages (like a slightly higher income).'